Answer: DEFINITELY A. The others make absolutely no sense.
Explanation:
Our Solar System has nine major planets. Only one is knows to have intelligent life.
Or if it must be in one sentence:
Our Solar System has nine major planets, but only one is knows to have intelligent life.
Dashes should be place around "the crowd was ecstatic" because it is an interruption. So "Yesterday's basketball victory - the crowd was ecstatic - guarantees us a chance at the state title.